Design and optimization of a combined supercritical CO2 recompression Brayton/Kalina cycle by using solar energy and LNG cold energy

Abstract

Solar energy and LNG cold energy have broad application scenarios as two renewable sources, but their complementary potential has not been fully explored. To fill this gap, this paper constructs a novel combined cycle that utilizes solar energy and recovers LNG cold energy to generate power. This combined cycle consists of a supercritical carbon dioxide recompression Brayton cycle, an ammonia-water mixture Kalina cycle, and an LNG cryogenic power system. A systematic evaluation was conducted on the thermodynamics and exergoeconomic of the model, and the simulation results under basic operating conditions show that the first law efficiency, second law efficiency, net power output and total unit cost reach 55.73 %, 47.72 %, 13.709 MW and 27.44 $·GJ−1 respectively. The genetic algorithm and elitist non-dominated sorting genetic algorithm are used to optimize the system performance. By using the parallel direct search method, the maximum first law efficiency, as well as second law efficiency and a minimum total unit cost of 59.45 %, 52.23 % and 24.81 $·GJ−1 are obtained respectively. The optimal first law efficiency, second law efficiency, and total unit cost of the system, which are 53.52 %, 50.33 % and 26.44 $·GJ−1 respectively, are searched simultaneously from the multi-objective optimization results by LINMAP decision-making method. Furthermore, this paper analyzes the exergy flow rate of the system and heat transfer processes of the heat exchangers under optimal operating condition. A total of 23,059.35 kW of LNG cold energy is fully utilized by the system in the LNG gasification process. The exergy loss ratio and cost ratio of each equipment analysis demonstrates that the heat exchangers (HX) have the largest percentage both exergy loss ratio and cost ratio, especially the HX4.